Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
As doubt cannot remove certainty, your fast is valid till you are sure that your menstruation has started.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
You need not bother about the doubt. You only need to consider your fast broken if you could ascertain the fact that menses did start earlier. Since you are in doubt, you should dismiss it.
According to the rules of jurisprudence, “Certainty is not removed by doubt”. In other words, you deem yourself free from menses until you are certain that bleeding did start.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.